The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to a local coil for acquiring NMR images of a selected part of a subject such as the human brain.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance ("NMR") phenomena is exploited.
The Larmor frequency of the NMR signals is determined by the magnitude of the polarizing magnetic field B.sub.0 and one of the design objectives of an MRI system is that it produce a homogeneous polarizing magnetic field throughout a volume of a specified size (e.g., a 50 cm sphere). To this end it is well known in the art to use shims which alter the magnetic field in order to make it homogenous. Such shimming may be accomplished actively by using shim coils which conduct the appropriate current are described in U.S. Pat. Nos. 4,949,043; 4,862,087 and 5,490,509. In the alternative, shimming may be accomplished passively by judiciously placing carbon steel shims at locations within the polarizing magnetic field as described in U.S. Pat. Nos. 5,677,854; 5,532,597 and 5,349,297. Such shimming is performed as part of the system calibration procedure and it is performed without a subject in the bore of the magnet.
It is well known in the art that the SNR of images produced by MRI systems can be increased by using small RF coils which are designed to couple solely with the tissues in the particular region of interest. Such "local" or "surface" coils have been designed for various parts of the human anatomy, such as knees, shoulders, neck, breasts, hands and head. Of particular relevance to the present invention are local head coils which employ the so-called "bird cage" RF coil described by C. E. Hayes et al, J. Magn. Reson. 63, 622-628 (1985) and U.S. Pat. No. 5,372,137.
The concept of acquiring NMR image data in a short time period has been known since 1977 when the echo-planar pulse sequence was produced by Peter Mansfield (J. Phys. C.10:L55-L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a set of NMR signals for each RF excitation pulse. These NMR signals can be separately phase encoded so that an entire scan of 32, 64 or 128 views can be acquired in a single pulse sequence of 20 to 100 mili-seconds in duration. The advantages of echo-planar imaging ("EPI") are well-known, and there has been a long felt need for apparatus and methods which will enable EPI to be practiced in a clinical setting. Other echo-planar pulse sequences are disclosed in U.S. Pat. Nos. 4,678,996; 4,733,188; 4,176,369; 4,355,282; 4,588,948 and 4,752,735.
Functional magnetic resonance imaging (fMRI) is a method to map human brain function. See for example, U.S. Pat. No. 5,603,322. This method is dependent on the fact that highly localized changes in blood flow, blood volume and blood oxygenation occur that are a consequence of brain neuronal activity. The method of blood oxygen level dependent (BOLD) imaging has been found to be particularly effective in fMRI. Gradient-recalled (GR) rather than spin echo (SE) recalled pulse sequences are required for optimum sensitivity to BOLD contrast. As a result, the majority of fMRI studies to date have been performed using GR EPI.
The images acquired with EPI pulse sequences are very sensitive to local B.sub.0 field inhomogeneities that arise from susceptibility differences between bone, tissue and air when a subject is placed in the otherwise homogeneous B.sub.0 field. At high fields such as 3 Tesla where functional MRI is practiced, a 1 ppm deviation in B.sub.0 magnetic field strength translates to a misplacement of 5 pixels in the phase encoding direction. It has become apparent, therefore, that further shimming must be performed after the subject to be imaged is placed in the MRI system.
One solution to this problem is to provide active shim coils that are located close to the subject as described in U.S. Pat. No. 6,023,167. The shim coils are integral with the local RF coil and a procedure is conducted after the subject is in place to determine the amount of current to apply to each shim coil to offset B.sub.0 field inhomogeneities. This solution requires many shim coils, each with a separate power supply. This is not only expensive, but the wiring needed to couple the shim coils to the remotely located power supplies is very cumbersome.